Method of producing electrically conductive article



Dec. 24, 1957 A. s. LOUIS ,81

METHOD OF PRODUCING ELECTRICALLY CONDUCTIVE ARTICLE Filed March 4, 1953 INVENTOR.

' ARNOLD S. LOUIS AGENT United States aren't Chloe 2,817,603 Patented Dec. 24, 1957 METHOD OF PRODUCING ELECTRICALLY CONDUCTIV E ARTICLE Arnold S. Louis, New York, N. Y., assignor to Myron A. Coler, Scarsdale, N. Y.

Application March 4, 1953, Serial No. 340,313

17 Claims. (Cl. 117-211) This invention relates to electrical insulators having electrically conductive surfaces and method of producing same.

The following is a continuation in part of my copending application, Serial No. 256,516 entitled Improved Electrically Conductive Articles and Method of Making Same, filed November 13, 1951, now U. S. Patent No. 2,788,297.

In many applications for electrical insulators such as glass and plastics it is desirable that an electrically conductive surface be provided in order to eliminate electrostatic charges. In electrical measuring apparatus electrostatic charges cause disturbances to sensitive meters. Accordingly, a conductive surface is sought for the meter casing observation window.

In another typical case, that of aircraft, the electrostatic charging of the plastic cockpit canopy and insulating plastic surfaces covering radio and radar antennas create electrical interference with radio communication and radar signals. To the best of my knowledge, prior to my invention no satisfactory solution to this problem existed. in typical attempts at a solution many faults were present. For example, the coating reduced the transmission of light, introduced an optical haze effect or optically distorted the pilots vision or the process of applying the coating resulting in unevenness and scratching of the delicate plastic surface. The process of this invention permits the production of complex shaped plastic objects having the conductive surface with little reduction in light transmission qualities of the plastic. Upon consideration it may be appreciated that it is indeed surprising that an adherent coating formed of an electrically conductive material may be applied to a highly polished, relatively soft plastic surface Without seriously affecting the light transmission and without causing scratches and pitting of the surface.

Again, it is often desirable that a coating of considerable conductivity, but not usually transparent, be applied to an insulator to serve as the base for electroplating. Yet again, a firmly adherent, highly uniform coating of conductive material on an insulator sheet can serve as an electrical plotting board or for related electronic applications. Furthermore, a conductive coating applied to an insulator surface can be used as a resistive heater element to maintain the insulator or its surroundings at adesired temperature.

There is disclosed herein a simple method of coating an insulator surface with conductive material involving repeatedly impacting the insulator surface with the con ductive material.

The prior art shows various partially successful methods of applying carbon and other conductive films. Thus, surfaces possessing suificient inherent adherence such, for instance, as the surface of a wax object, will readily attach to itself a relatively continuous graphite layer which can be electroplated. ,Suchfilms are rather delicate and usually completely opaque. Again, it is conventional to apply graphite in dispersion in filmwfp rming adhesives to the surfaces of'insulator objects. It has not been possible to obtain such coatings which are sufficiently conductive and, at the same time, reasonably transparent. Furthermore, there are problems connected with peeling and loose adherence of the coating and with crazing of insulator plastics to which they are applied.

A conventional device to obtain transparent conductive coatings is the application of organic films containing water-soluble electrolytes. Such films are generally very unstable as to their electrical properties and can be easily washed from the insulator base. This invention, by contrast, is concerned with obtaining a coating which is sufficiently thin to be transparent and yet sufiiciently adherent to withstand considerable abrasion and washing.

Accordingly, it is an object of this invention to provide an improved method for applying an electrically conductive coating to the surface of an electrically non-conductive article.

It is another object of this invention to provide a method for applying an electrically conductive coating to the surface of an electrically non-conductive plastic article.

A still different object is to provide a glass having an electrically conductive surface.

A particular object of this invention is a method of obtaining a uniform conductive coating on a plastic surface.

Still another object of this invention is to provide a method of applying a transparent electrically conductive surface to electrically non-conducting articles.

Another particular object of this invention is to provide a convenient method for electrically heating the surface of objects made of electrically non-conducting articles.

Gther objects and advantages of this invention will appear more fully and clearly from the following description of illustrative embodiments thereof taken in connection wth the appended drawings in which:

Figure 1 shows partially in section, a plan view of a typical impactor described herein.

Figure 2 shows in plan an apparatus for carrying out the teachings of this invention.

Figure 3 presents pictorially another embodiment of this invention.

Figure 4 presents a partially sectioned elevation view of a vibratory apparatus for carrying out the teachings of this invention.

Figure 5 shows partially schematically and partially pictorially an improved burglar alarm system of this invention.

The process of this invention comprises repeatedly impacting the substrate to be coated with finely divided electrically conductive material. Finely divided particles, in general, lack enough mass to hit the substrate with sufficient energy to become firmly attached to the surface. In carrying out the process of this invention the finely divided coating material is temporarily afilxed to a carrier particle having considerably greater mass, and certain other properties pointed out later, and cansing the coated carrier particles and the article to be coated to collide frequently. This may be accomplished by movement of the article or by hurling the carrier particle at the substrate so as to utilize its kinetic energy to hammer the finely divided particles onto the substrate. The term impactor hereafter refers to the carrier.

The design of the impactor particles will depend in part on the nature and sizeof the surface to be coated on, the particular method by which the impactors are to be thrown against the insulator surface, and on the amount of coating which it is desiredto apply.

In general, it is preferred that the impactor particles be round rather than angular in shape to avoid scratching of the insulator surface. Larger and heavier impactors are preferred, the harder the surface to be coated and the higher the conductivity which it is desired to impart to the treated surface. In some situations it has been found satisfactory to use soft materials as impactors, particularly for use with extremely soft surfaces such as plastics.

Thus, polystyrene beads about 1 mm. in diameter have been found effective in coating polymethylmethacrylate sheet. Steel balls, A; inch in diameter, have been effective in applying conductive coatings to glass and have also been used to apply rather heavy conductive coatings to polymethylmethacrylate. Impactors formed of poly ethylene balls, /8" in diameter, have been found effective in applying conductive coatings to the surface of polymethylmethacrylate where the objective was to obtain a fairly thin and transparent electrically conductive coating.

In general, impactors should not be smaller than 0.0025 inch nor larger than 0.25 inch. Smaller impactors lack the necessary mass to be effective. With larger impactors it is inconvenient to obtain the multitudinous impactions which are needed to secure the full advantage of this invention.

Impactor particles may be made of any material which is stable physically and chemically under the conditions of use. They should not have such an adhesive surface as to bond the conductive material inextricably to themselves. Suitable impactor materials include glass, polylstyriene, steel balls, ceramics, polyethylene, rubber and The electrically conductive coating material must be finely divided and chemically stable under conditions of impact. The range of fineness is critical and the particles should not be coarser than ten microns and preferably finer than two microns in the smallest dimension. Particles of laminar habit are particularly convenient for the practice of this invention.

Impactor particles may be coated with conductive material by any of several techniques, as for instance, by exposure to the sooty products of the incomplete combustion of carbonaceous gaseous fuels or by wetting with a suspension of conductive material in a liquid followed by drying of the coated particles. The preferred method mvolves tumbling the impactors with finely divided conductive material until a uniform coating is obtained.

Only a small amount of conductive material, usually 0.01 to 0.5% by weight of the quantity of impactor used is needed. After impactors have been used for some time it will be necessary to replenish the coating of conductive material.

I have found that if these coated impactors are hurled at suitable types of surfaces the finely divided material will firmly adhere to the receiving surface. I have used various methods for imparting the required velocity to the impactors. These methods include use of a high velocity fluid medium such as air, a form of tumbling technique, dropping the impactors from a height above the article to be coated and the use of a vibratory technique to cause the impactors, article to be coated, or both, to repeatedly move into contact with the other such as by the use of vibratory devices. It is often preferred to suspend an article to be coated in a container filled with the coated impactors of this invention. The support for the article is connected to a source of reciprocating motion or vibration. A rapid, random vibration of small amplitude is preferred. Thus a vibrator having a motion of about 0.040 inch and a frequency of 60 cycles per second has been found to be particularly useful. Vibration of small amplitude and high frequency is particularly useful for applying uniform coatings to articles of complex shape.

a spherical bead of polystyrene 4 coated with finely divided particles of graphite.

It is within the scope of this invention to coat glass articles with a thin plastic surface to which is then applied the electroconductive coating of this invention.

For purpose of illustration and without intending it tov be limiting, the solvent coating may be a silicone resin such as Dri-Film, a product of the Dow Corning Corp, a cast film of polymethylmethacrylate applied from solution or other adherent plastic coating.

In Figure 2 there is disclosed a method suitable for the coating of large irregularly shaped articles. Container 52 contains a quantity of coated impactors 54 into which is immersed a plastic article 56, such as an airplane cockpit canopy. A vibratory motor 58 is attached to the canopy 56 so as to rapidly vibrate it through the impactors 54.

In Figure 3 there is disclosed another method of coating an article. Container 60 contains a quantity of impactors 62 which are coated with an electrically conductive material and into which is immersed a plastic article 64 which is to be coated. The article is fastened to arm 66 of an oscillatory mechanism such as a jig saw 68 which is adapted to move plastic article 64 rapidly up and down in the bath of coated impactor particles 62. A coating of conductive material is thus formed on the plastic article.

In Figure 4 a modified laboratory sieving apparatus is shown having vibratory base 70 on which are mounted solid pans 72 in place of the normal sieves as containers for coated impactors 74. Articles 76 to be coated are then introduced into the containers and the apparatus permitted to vibrate. This apparatus is particularly useful for the treatment of irregularly shaped articles.

It has been found that considerable improvement in adhesion is obtained if the surface of the coated plastic is solvent treated. This treatment may be applied by dipping the coated article into a plastic bath or by spraying the surface with solvent.

In Figure 5 there is shown a typical plate glass window 80 having a transparent conductive strip 82 applied thereon in accordance with the teachings of this invention. The coating is applied in the form of a continuous band and is obtained by masking oft areas of the glass prior to coating. In the conventional burglar alarm system an unsightly metal strip in the form of a border is cemented onto the glass. The strip is placed in series connection with a current source 84 and an alarm 86 actuated by a disruption in the current. Burglars often cut an opening in the center of a store window and thus remove the contents without breaking the bordering conductive strip. Being transparent the coating may be applied in a pattern covering the entire window without interfering with visibility. The treated window of this invention can not be tampered with in this fashion without breaking the circuit and setting off the alarm.

For a better understanding of the invention a number of specific examples are presented hereafter.

Example 1 A 10 inch diameter jar mill of 1 gallon capacity was charged with 2000 grams of XXX grade polystyrene beads made by Koppers Co., Inc. and 0.5 gram of 200-10 graphite made by Dixon Crucible Co. The beads had an average diameter of 1 mm. The mill was rotated for five hours at 70 R. P. M.

A 1000 cc. beaker was filled three-quarters full with the prepared impactors. A sheet of plastic, 1 inch x 6 inches x inch, fastened to the arm of electrically operated jig saw, was then introduced into the beaker. The jig saw was run with a stroke of 1 inch and a speed of 1000 cycles per minute for a period of 1 minute. When removed the sheet was found to have a coating with a light transmission of 85 percent and a resistance of 0.7

The typical impactor 2, shown in Figure 1, consists of 76 megohm P q Example 2 An apparatus conforming to that described in Figure 4 which utilized pans 8 inches in diameter was charged three-quarters full with impactors coated in conformity with those of Example 1. Polyrnethylmethacrylate balls 1 inch in diameter were introduced into the container and the unit permitted to vibrate for a period of 3 hours, the end of which time the polymethylmethacrylate balls were moved and it was found that a uniform coating was obtained. This experiment indicates the complex shapes which may be uniformly coated.

Example 3 Example 2 was repeated. The coated sheet was then immersed for seconds in a solution of a solvent consisting of equal parts of methyl ethyl ketone and ethylene glycol monoethyl ether acetate, removed and air dried in a position which permitted rapid draining. No reduction in visibility or light transmission was noticeable; however, under abrasion carried out by rubbing briskly with a cloth it was found that the coating after such solvent treatment was considerably more adherent.

The choice of solvent to be used is dictated by the characteristics of the plastic; for example, the solvent chosen should not be one which will cause crazing of the plastic surface nor undue swelling.

Example 4 A 1000 cc. beaker was filled three-quarters full with coated impactors conforming to those used in Example 1. A sheet of polymethylmethacrylate 1 inch wide and 8 inches long bent in approximately the shape of a question mark was attached to a vibratory tool operating at a frequency of about 60 cycles per second and an amplitude of about 0.040 inch. The plastic piece was immersed in the coated beads and vibrated for 2 minutes. The highest resistance for any 4 inch length of the specimen was 2.0 megohms; the lowest 0.5 megohm.

Example 5 2500 grams of lead shot, 0.030 inch in diameter and 0.17 gram of graphite (as used in Example 1) were charged to a 500 cc. bottle and tumbled at 40 R. P. M. for 3 hours. A sheet of polymethylmethacrylate 2 inches x 2 inches x inch was then put in the bottle and tumbling was continued for 30 minutes. The plastic sheet was found to have a coating with a light transmission of 65% and a resistance of 30,000 ohms per square.

Although it has been pointed out that adhesives are not required in carrying out this invention it is to be noted that pretreatment of the surface before coating may be helpful under some circumstances. Such treatment may include the cleaning of the surface or the heating of the surface until it is tacky.

In carrying out the invention the atmosphere within the tumbling barrel may be adjusted to meet the re quirements of the particular combination of the coating material and article to be coated such as hot or cold temperature ranges and/or reducing gas atmosphere to prevent oxidation of the various surfaces.

Graphite compacts or chips may be used as impactors which are capable of transferring fine particles of their own substance to a surface which they impact.

Large complex shaped plastic articles such as aircraft cockpit canopies, bomber nose bays and the like are usually made by clamping a polymethylmethacrylate sheet around its outer edge to a container so as to form an airtight seal. The plastic sheet is then heated beyond its heat distortion point and by means of compressed air blown up, bubble fashion, into free space or against a shaping form. It is customary practice to trim off the flange upon completion of the operation. I prefer to temporarily retain the flange and to use it for purpose of clamping the article to be coated into the vibratory apparatus of Figure 2 in such a way that the impactors are sealed into the apparatus. One way to accomplish this is to secure a rubber sheet to the flange and to the tank rim. The rubber covering will in no way interfere with the operation and yet will keep impactors from the interior of the canopy. This eliminates the need for otherwise protecting the interior of the canopy. v

The electroconductive coatingof this invention makes possible an improved process for uniformly coating electrical insulators with finishing materials. The canopy coated, as for example, by the best presently known em bodiment of this invention, as exemplified by experiment 1 and shown in Figure 2, may be electrostatically charged and then coated by a mist like spray of an electrically charged transparent finishing material such as a solution of a copolymer of methacrylic acid and methylmethacrylate thus providing a protective coating. Electrostatic coating equipment is commercially available and need not be described here. The process of coating electrically non-conductive articles is believed novel as heretofore the apparatus has been employed only for coating metal surfaces.

Other applications for this invention include fluorescent lighting fixtures of the type wherein a transparent sheet having an electroconductive surface is made one electrode of a capacitor having a fluorescent material as a, dielectric. Upon application of an electric potential to the capacitor plates the dielectric material glows.

The term plastic" as used herein embraces any one of a large and varied group of materials commonly referred to, among chemists and chemical engineers, as plastics, and resins, such as polymethylmethacrylate, po'lyvinylidene chloride, polyethylene, polystyrene, polyvinylchloride and their copolyme-rs.

Although preferred embodiments have been disclosed, it will be understood that modifications may be made within the spirit and scope of the invention.

I claim:

1. The process of rendering the surface of plastic articles electrically conductive comprising repeatedly impinging said surface with impactors, said impactors having at least a portion of their surface coated with finely divided electrically conductive material; and treating only the surface of the resulting electrically conductive surface with a solvent for said plastic.

2. The process of rendering the surface of plastic articles electrically conductive comprising coating the said surface with electrically conductive particles of graphite, wetting only the surface of said articles with a solution which is a solvent for said plastic and drying said article.

3. The process of rendering the surface of polymethylmethacrylate articles electrically conductive comprising coating the said surface with electrically conductive particles of graphite, wetting only the surface of said articles with a mixture of methyl ethyl ketone and ethylene glycol monoethyl ether acetate and drying said article.

4. The method of applying an electrically conductive coating of finely divided solid non-fusible conductive material having an average particle size of less than 10 microns, to an electrically non-conductive substrate comprising the steps of at least partially coating substantially spherically shaped impactors having a particle size in the range between 0.0025 and 0.25 inch with said finely divided conductive material, surrounding said substrate with a plurality of said coated impactors and vibrating said impactors and said substrate relative to each other so as to cause said impactors to repeatedly impinge upon the surface of said substrate and thereby transfer particles of said conductive material from said impactors to said substrate so as to form a conductive coating on said substrate.

5. The method of claim 4 wherein said substrate is transparent.

6. The method of claim 5 wherein said transparent substrate is selected from the group consisting of trans- 7 parent synthetic resins and. transparent cellulose acetate.

7. The method of claim 4 wherein said finely divided. conductive material is graphite.

8. The method of claim 5 wherein said finely divided conductive material is graphite and said coated step is discontinued after a transparent conductive coating is formed.

9. The method of claim 6 wherein said finely divided conductive material is graphite and said coated step is discontinued after a transparent conductive coating is formed.

10. The method of claim 5 wherein said substrate is polymethyl methacrylate and said conductive material is graphite and said coated step is discontinued after a transparent conductive coating is formed.

11. The method of applying an electrically conductive coating of finely divided solid non-fusible conductive material having an average particle size of less than 10 microns, to an electrically non-conductive substrate comprising the steps of at least partially coating substan tially spherically shaped impactors having a particle size in the range between 0.0025 and 0.25 inch with said finely divided conductive material, surrounding said substrate with a plurality of said coated impactors, maintaining said substrate in a fixed position, and vibrating said impactors relative to each other and to said substrate so as to cause said impactors to repeatedly impinge upon the surface of said substrate and thereby transfer particles of said conductive material from said impactors to said substrate so as to form a conductive coating on said substrate.

12. The method of claim 11 wherein said substrate is transparent.

13. The method of claim 12 wherein said transparent substrate is selected from the group consisting of transparent synthetic resins and transparent cellulose acetate.

14. The method of claim 11 wherein said finely divided conductive material is graphite.

15. The method of claim 11 wherein said finely divided conductive material is graphite and said coated step is discontinued after a transparent conductive coating is formed.

16. The method of claim 13 wherein said finely divided conductive material is graphite and said coated step is discontinued after a transparent conductive coating is formed.

17. The method of claim 12 wherein said substrate is polymethyl methacrylate and said conductive material is graphite and said coated step is discontinued after a transparent conductive coating is formed.

References Cited in the file of this patent UNITED STATES PATENTS 1,385,184 Meade et al. July 19, 1921 1,923,406 Wiegand Aug. 22, 1933 2,326,001 Ariotti Aug. 3, 1943 2,554,723 Webb May 29, 1951 2,618,572 Parrish Nov. 18, 1952 OTHER REFERENCES Printed Circuit Techniques, Natl Bureau of Standards, Nov. 15, 1947, Circular 468, pages 5-9. 

4. THE METHOD OF APPLYING AN ELECTRICALLY CONDUCTIVE COATING OF FINELY DIVIDED SOLID NON-FUSIBLE CONDUCTIVE MATERIAL HAVING AN AVERAGE PARTICLE SIZE OF LESS THAN 10 MICRONS, TO ELECTRICALLY NON-CONDUCTIVE SUBSTRATE COMPRISING THE STEPS OF AT LEAST PARTIALLY COATING SUBSTANTIALLY SPHERICALLY SHAPED IMPACTORS HAVING A PARTICLE SIZE IN THE RANGE BETWEEN 0.0025 AND 0.25 INCH WITH SAID FINELY DIVIDED CINDUCTIVE MATERIAL, SURROUNDING SAID SUBSTRATE WITH A PLURALITY OF SAID COATED IMPACTORS AND VIBRATING SAID IMPACTORS AND SAID SUBSTRATE RELATIVE TO EACH OTHER SO AS TO CAUSE SAID IMPACTORS TO REPEATEDLY IMPINGE UPON THE SURFACE OF SAID SUBSTRATE AND THEREBY TRANSFER PARTICLES OF SAID CONDUCTIVE MATERIAL FROM SAID IMPACTORS TO SAID SUBSTRATE SO AS TO FORM A CONDUCTIVE COATING ON SAID SUBSTRATE. 